Currently, the most common solution to constant-current control of multipath light-emitting diodes (LEDs) includes a constant-voltage module and multiple non-isolated DC/DC constant-current modules.
FIG. 1 shows a constant-current control circuit for multipath LEDs in the prior art. In this circuit, an input voltage Vac goes through a constant-voltage module and then inputs to multiple non-isolated DC/DC constant-current modules. Each of the non-isolated DC/DC constant-current modules performs constant-current control separately. However, there is normally a significant disparity between the voltage of the constant-voltage module and the voltage of an LED load; therefore none of the non-isolated DC/DC constant-current modules that follow the constant-voltage module has a high efficiency. In addition, the structure of multiple non-isolated DC/DC constant-current modules is complex and costly.
According to the Chinese patent application No. 200810085227.5, a balanced-current power supply circuit for multiple groups of LEDs is provided. As shown in FIG. 2, a first inductor Lca1 of a coupled inductor Lca is connected in series to a Direct current (DC) loop after the rectification by diodes D1 and D2, and a second inductor Lca2 of the coupled inductor Lca is connected in series to a DC loop after the rectification by diodes D3 and D4, so that the coupled inductor Lca can balance the two LED loads. However, according to the circuit shown in FIG. 2, each of the two coils of the coupled inductor Lca is connected in series to a DC loop, causing a DC current, but the magnetizing current in the coupled inductor is unidirectional; therefore when the voltages of the two load branches are not balanced, the difference between the currents of the two load branches is large, resulting in poor current balancing. Moreover, the presence of a DC current in the coupled inductor may cause saturation of the magnetic core, which requires air gaps to be created; therefore when the inductance is large, the size of the coupled inductor is large and hence costly.